Pressure relief detection for use with gas storage

ABSTRACT

The present disclosure is directed to a method and system for detecting activation of a pressure relief device connected to a storage tank containing a pressurized gas. The method includes calculating a pressure relief device release rate based on a set of inputs, wherein the set of inputs includes at least one of a storage tank volume, a pressure relief set point, an orifice size of the pressure relief device, a gas density, and a reseat point for the pressure relief device. The method further includes monitoring the pressure within the storage tank and calculating a differential pressure reading over time, comparing the differential pressure reading over time to the pressure relief device release rate, and detecting a pressure relief device activation based on the comparison result.

This application claims the benefit of U.S. Provisional Application No.61/869,119, filed Aug. 23, 2013, which is incorporated herein byreference.

The present disclosure is directed towards pressure relief detection foruse with gas storage, and more specifically, to detection andnotification of activation of a pressure relief device.

Safe and effective gas storage requires various technologies to monitora gas within a storage device. Some gases and storage systems requirespecific safety devices, depending on where and how a gas is stored. Gasusage may also require consideration. For example, hydrogen storage isrequired for the successful commercialization of hydrogen and fuel cellpower technology in transportation, stationary, and portableapplications. The long-term sustainability of a “hydrogen economy”depends largely on the efficiency, safety, and cost-effectiveness ofhydrogen storage. Gaseous hydrogen is a convenient and common form ofstored energy, usually by pressurized containment. Advantageously,storing hydrogen at high pressure yields higher energy density.

Hydrogen storage tanks can fall under four classifications, Type 1, Type2, Type 3, and Type 4. Each tank has a different maximum pressurecapacity based on the materials of construction; Type 1 is rated for thelowest pressure and Type 4 is rated for the highest pressure. Forexample, a Type 1 tank made of steel can have a maximum pressure ratingof about 2,900 psi, while a Type IV tank made of plastic/carbon can havea maximum pressure rating of equal to or greater than 9,500 psi. In thelast decade Type IV tanks have been used for hydrogen storage aboardfuel cell powered automobiles manufactured by several major automakers.

Hydrogen storage tanks for use in stationary and mobile vehicles requirean integrated pressure relief valve (PRV) configured to preventover-pressurization of the tank and supporting components in the eventof a control element failure, fire, or blocked flow. Typically thesePRVs are required to be American Society of Mechanical Engineers (ASME)or Department of Transportation (DOT) certified depending on the tank'sapplication. One challenge with regard to simple PRVs is a lack ofability to detect and have a control system react appropriately in theevent of a PRV activation caused by over-pressurization. The ability fora control system to detect activation could enable specific controlsystem reactions, for example, in the case of a bulk storage system; acontrol system reaction can prevent supplying additional hydrogen to thestorage system. In the case of a mobile application, the control systemreaction can stop the consumption of fuel and have the vehicle reactappropriately. In addition, the ability to detect activation by thecontrol system can allow the control system to send notification to acustomer, user, or service personnel.

Lately, more intelligent PRVs have been developed, which use integratedproximity switches to detect the activation of the PRV. However, thesePRVs are more complicated, costly, and not available for high pressureapplications. Currently, the intelligent PRVs available are not ratedfor high pressure applications (i.e., pressures greater than 5,000 psi).In addition, reliability is a concern, because these PRVs can give falseactivations. Improved systems, devices and methods are thus needed todetect activation of pressure relief in gases stored at high pressures.

In consideration of the aforementioned circumstances, the presentdisclosure is directed towards a new method and system for pressurerelief device activation detection for use with pressurized gas storage.The method and system are configurable for specific gases used in highpressure applications.

One embodiment of the present disclosure is directed to a method ofdetecting activation of a pressure relief device connected to a storagetank containing a pressurized gas. The method comprises calculating apressure relief device release rate based on a set of inputs, whereinthe set of inputs includes at least one of a storage tank volume, apressure relief set point, an orifice size of the pressure reliefdevice, a gas density, and a reseat point for the pressure reliefdevice. The method further includes monitoring the pressure within thestorage tank and calculating a differential pressure reading over time,comparing the differential pressure reading over time to the pressurerelief device release rate, and detecting a pressure relief deviceactivation based on the comparison result.

In another embodiment, the calculation of the pressure relief devicerelease rate is based on choked flow equations for the pressure reliefdevice. In another embodiment, calculating the differential pressurereading over time is repeated and the differential pressure values areaveraged over time. In another embodiment, the method further comprisesactivating a system alarm following detection of the pressure reliefdevice activation.

In another embodiment, detecting the pressure relief device activationbased on the comparison result comprises identifying when thedifferential pressure reading over time is greater than or equal to thepressure relief device release rate. In another embodiment, calculatingthe pressure relief device release rate further comprises factoring inthe position of a regulating device configured to discharge thepressurized gas during normal operation.

Another embodiment of the present disclosure is directed to a controllerfor detecting activation of a pressure relief device connected to astorage tank containing a pressurized gas. The controller comprises aprocessor configured to receive an input of the pressure reading from apressure transducer, calculate a differential pressure reading over timeand compare the differential pressure reading over time to a pressurerelief device release rate determined based on a set of inputs, andbased on the comparison the controller detects whether a pressure reliefdevice activation has occurred, wherein the pressure transducer islocated upstream of the pressure relief device and the pressure withinthe storage tank is about greater than about 10,000 psi. In anotherembodiment, the controller further comprises a graphical user interface,a memory device, and a power source. In another embodiment, the set ofinputs includes at least one of a storage tank volume, a pressure reliefset point, an orifice size of the pressure relief device, a gas density,and a reseat point for the pressure relief device.

Another embodiment of the present disclosure is directed to a systemconfigured to detect pressure relief. The system comprises at least onestorage tank configured to contain a gas, a pressure transducerconfigured to read the pressure of the gas, a pressure regulating deviceconfigured to control the discharge of the gas during normal operation,a pressure relief device configured to activate and discharge the gas toprevent an over-pressurization of the storage tank, and a controllerconfigured to receive the pressure reading from the pressure transducer,calculate a differential pressure reading over time and compare thedifferential pressure reading over time to a pressure relief devicerelease rate determined based on a set of inputs, and based on thecomparison the controller detects whether the pressure relief device hasbeen activated.

In another embodiment, the controller further comprises a processor, agraphical user interface, a memory device, and a power source. Inanother embodiment, the set of inputs includes the storage tank volume,a pressure relief set point, an orifice size of the pressure reliefdevice, the gas density, and a reseat point for the pressure reliefdevice. In another embodiment, the calculation of the pressure reliefdevice release rate is based on choked flow equations for the pressurerelief device. In another embodiment, calculating the differentialpressure reading over time is repeated and the results are averaged overtime. In another embodiment, the controller is further configured toactivate a system alarm following detecting the pressure relief deviceactivation, and the system alarm is configured to provide notificationto at least the proper personnel or system to respond. In anotherembodiment, the controller is configured to factor in the position of aregulating device configured to discharge the pressurized gas duringnormal operation when calculating the pressure relief device releaserate. In another embodiment, the pressure relief device orificecross-sectional area is greater than the cross-sectional area of thepressure regulating device orifice.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentdisclosure and together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 is a schematic diagram of part of a storage system, according toan exemplary embodiment.

FIG. 2 is a schematic diagram of part of a pressure relief device,according to an exemplary embodiment.

FIG. 3 is a flow diagram for control logic, according to an exemplaryembodiment.

Reference will now be made in detail to the present exemplaryembodiments of the present disclosure, examples of which are illustratedin the accompanying drawings. Wherever possible, the same referencenumbers will be used throughout the drawings to refer to the same orlike parts. Although described in relation to hydrogen storage, it isunderstood that the methods and systems of the present disclosure can beemployed with various types of pressurized storage systems. For example,pressurized gas storage of natural gas, nitrogen, carbon monoxide,helium, argon, carbon dioxide, chlorine, oxygen, and other like gas.

FIG. 1 shows a portion of a storage tank system 100, according to anexemplary embodiment. Storage tank system 100 can comprise a storagetank 110, an inlet passage 120, an outlet passage 130, a regulatingdevice 140, a pressure relief device 150 (PRD), a pressure transducer160, a controller 170, and an exhaust passage 180.

Storage tank 110 can be configured to receive, store, and discharge agas 111. Gas 111 can include an energy source, such as, for examplehydrogen gas or natural gas. Storage tank 110 can be formed of metal(e.g. steel or aluminum), plastic, composites, carbon fiber, or othermaterials. Storage tank 110 can have a maximum pressure capacity ofabout 5,000 psi, 10,000 psi or 15,000 psi and above. Storage tank 110can have a volume of about 1 liter, 2 liters, 5 liters, 10 liters, 50liters, 100 liters, 1,000 liters, 10,000, or 50,000 liters and above.Storage tank 110 can be formed of a single chamber or a plurality ofchambers (not shown). The plurality of chambers can be isolated byvalves or in fluid communication. Within the plurality of tanks gas canbe stored at different pressures. For example, there can be low pressurestorage tanks, medium pressure storage tanks, and high pressure storagetanks. For example, for an industrial application, the low pressure tohigh pressure range can be about 500 psi to about 6,500 psi and for anautomotive application; the low pressure to high pressure range can beabout 2,000 psi to about 15,000 psi.

Inlet passage 120 can be configured to allow gas 111 flow into storagetank 110. Inlet passage 120 can be configured to be a single inlet or aplurality of inlets. For a bulk storage system, inlet passage 120 can beconfigured to be in fluid communication with a gas generation supplysource (not shown). For example, in the case of a hydrogen storagesystem, inlet passage 120 can be in fluid communication with a hydrogencompression system (e.g., electrochemical hydrogen compressor), highpressure electrolyze, or other similar gas compressor or source. For amobile vehicle application, inlet passage 120 can be configured toreceive a refueling line connection, such as, for example, a refuelingnozzle (not shown).

Outlet passage 130 can be configured to discharge gas 111 from storagetank 110. For a bulk storage system application, outlet passage 130 canbe the refueling connection used to refuel the storage tanks within amobile vehicle. For a mobile vehicle application, outlet passage 130 canbe configured to feed a vehicle engine or fuel cell Outlet passage 130can also be configured to connect to a vehicle storage tank in the caseof a stationary storage application. In other embodiments, outletpassage 130 can be configured to operate as both the inlet passage andoutlet passage. In which case, a bypass (not shown) around regulatingdevice 140 can be configured to allow for reverse flow and a check valve(not shown) can be installed in the line.

Regulating device 140 can be located between storage tank 110 and outletpassage 130. Regulating device 140 can be configured to control thedischarge pressure and/or flow of gas 111 from storage tank 110. Forexample, regulating device 140 can take the form of a control valve(e.g., ball valve, butterfly valve, needle valve, gate valve, stem valveor diaphragm valve). Regulating device 140 can contain anorifice/opening, which cross-sectional area can vary based on thepercent open.

According to various embodiments, regulating device 140 can beconfigured to fully close and isolate gas 111 within storage tank 110from outlet passage 130. Regulating device 140 can be configured tocommunicate with controller 170. For example, controller 170 can receiveposition information from regulating device 140 (i.e., open, closed,percent open, etc.).

As shown in FIG. 1, pressure transducer 160 can be located betweenstorage tank 110 and regulating device 140 and be in fluid communicationwith both. Pressure transducer 160 can be configured to read a pressureassociated with gas 111 within storage tank 110. Pressure transducer 160can be configured to read the pressure of gas 111 continuously orperiodically. Pressure transducer 160 can be configured to continuouslyor intermittingly transmit the pressure reading to controller 170.

As shown in FIG. 1, downstream of pressure transducer 160 can be anauxiliary line comprising PRD 150 and exhaust passage 180. PRD 150 canbe configured to discharge gas 111 through exhaust passage 180 whenactive (i.e., open) and isolate exhaust passage 180 from storage tank110 when deactive (i.e., closed).

According to an exemplary embodiment, as shown in FIG. 2, PRD 150 cancomprise a body 151, an orifice 152, a disk 153, and a spring 154.Spring 154 can apply a force on disk 153 and block orifice 152. Theforce applied by spring 154 can be adjusted. PRD 150 can be configuredto prevent over-pressurization of storage tank system 100 by allowinggas 111 to flow out through exhaust passage 180 when the pressure of gas111 exceeds a predetermined pressure limit set point.

According to various embodiments, the cross-sectional area of orifice152 is greater than the cross-sectional area of the orifice/opening ofregulating device 140. Therefore, when the pressure limit set point isexceeded, flow through orifice 152 and out exhaust passage 180 can bethe path of least resistance.

According to various embodiments, when the pressure of gas 111 dropsbelow the pressure limit set point, the spring force can cause disk 153to reseat, blocking orifice 152 and stopping the flow of gas 111 throughPRD 150 and exhaust passage 180.

As shown in FIG. 1, according to an exemplary embodiment, controller 170can be comprised of a processor 171, memory device 172, and a graphicaluser interface (GUI) 173. Controller 170 can be configured to receiveinputs from instrumentation (e.g. sensors, valves, control devices,etc.) in addition to receiving programmed inputs from a user or operatorby way of the GUI 173. Controller 170 can be configured to monitorinstrumentation readings at a set interval, for example, every 1 second,0.1 seconds, 0.01 seconds, or 0.001 seconds. Communication betweencontroller 170 and pressure transducer 160, and regulating device 140can be by control wiring using current/voltage (e.g., 4-20 ma signal or1-10 VDC signal), field bus communication (e.g., PROFIBUS or MODBUS),pulse/frequency protocol, Wi-Fi, or Bluetooth®.

GUI 173 can allow a user to program a set of inputs that can include astorage tank 110 volume, a pressure relief set point, an orifice sizefor PRD 150, gas 111 density, and reseat point for PRD 150. The pressurerelief set point can be based on the maximum allowable operatingpressure of the system (i.e., storage tank, piping, components, etc.)According to various other embodiments, controller 150 can be configuredto detect at least one or more of the set of inputs directly from thedevices using smart instrumentation hardware configured to communicateusing a form of field bus communication. In yet another embodiment,controller 170 can include a programmable logic controller (PLC)configured to communication with a human machine interface (HMI) orother PLC on a common network and at least some of the inputs can bereceived as inputs from another controller or PLC.

According to an exemplary embodiment, a pressure relief device releaserate can be calculated for a given PRD 150. Choked orifice flowcalculations can be used to calculate a pressure relief device releaserate from a set of input variables.

The set of input variables can include storage tank 110 volume, pressurerelief set point, PRD 150 orifice size, gas 111 density, and reseatpoint for PRD 150. Based on these inputs a pressure relief devicerelease rate for PRD 150 can be calculated. Accordingly, if theeffective orifice size of PRD 150 is greater than the maximum effectiveorifice size of regulating device 140, then dP/dt during normaloperation (i.e., flow through regulating device 140) will be smallerthan release rate during a PRD 150 release. This can allow thecalculated release rate for PRD 150 to be used as an alarm or set pointin control logic to detect if PRD 150 has been activated.

As described above, according to an exemplary embodiment, controller 170can execute the calculations to determine the release rate value for PRD150. According to other embodiments, the calculation of the release ratevalue can be performed and then the value can be sent to controller 170as an input signal or programmed by a user through GUI 173.

EXAMPLE 1

The following example outlines the equations and steps that can be usedto calculate the pressure relief device release rate. For the followingexample the gas is assumed to be hydrogen. Table 1 shown belowidentifies the units used for the calculation and Table 2 identifies theindices.

TABLE 1 Unit Unit Identifier Measurement Description V m³ Volume P kPa,Pa, MPa Absolute pressure in the noted values T ° C., K Temperature inthe noted values ρ kg/m³ Real gas density ρ_(molar) Mol/L Real molardensity k Dimensionless Ratio of specific heats (Cp/Cv) Assumed to be1.41 for hydrogen at standard conditions Z Dimensionless CompressibilityFactor R J/mK Universal Gas Constant of 8.3144621 R J/kgK Specific GasConstant C Dimensionless Coefficient of Discharge A m² Area in meterssquared {dot over (m)} kg/sec Mass flow rate t Seconds Time Q m³/secVolumetric flow rate MW g/mol Molecular Weight π Dimensionless Pi -Ratio of a circles circumference to the diameter - 3.14156

TABLE 2 Indices Description Crit Critical i Inlet Condition o OutletCondition Ori Orifice c Choked conditions nc Non-choked conditionsVessel Hydrogen Supply Vessel Reseat Reseat Result Resultant initialInitial Condition final Final Condition release Conditions after apressure relief valve device activates

The process for calculating the pressure relief device release rate canbe described in several sections. The first section can comprisedetermining the input conditions for the calculations (i.e., vesselinitial conditions, critical pressure ration, orifice area), the secondsection can comprise the orifice equations (i.e., dischargecoefficients, choked flow equation, and non-choke flow equation), thethird section can comprise dynamically analyzing the PRD release (i.e.,determining the resultant vessel pressure after Time=i, determiningorifice conditions after Time=i, and iterative modeling), and the foursection can comprise of determining the PRD release rate set point.Lastly, the calculations can also factor in the additional gas consumersand suppliers.

Initial Vessel/Storage Tank Conditions:

The initial conditions can be defined as the state that the vessel is inbefore the pressure release occurs. The following equations can be usedto determine the initial vessel conditions.

Vessel/Storage Tank Volume:

V _(vessel) =V _(cylinder)×Number of Cylinders  (1)

Density of Hydrogen Gas:

Hydrogen does not behave in the ideal gas realm, therefore the real gasdensity needs to be calculated from the equation of state, see, e.g.,“Revised Standardized Equation of State for Hydrogen Gas Densities forFuel Consumption Applications,” E. W. Lemmon, M. L. Huber, J. W.Leachman, J. Res. Natl. Inst. Stand. Technol. 113, 341-350 (2008). Thisequation utilizes the ideal gas equation and corrects for real gas bycalculating the compressibility factor for a given pressure andtemperature condition.

$\begin{matrix}{Z = {1 + {\sum\limits_{i = 1}^{9}\; {{a_{i}\left\lbrack \frac{1000\mspace{14mu} K}{T} \right\rbrack}^{Bi}\left\lbrack \frac{P({MPa})}{1\mspace{14mu} {MPa}} \right\rbrack}^{Ci}}}} & (2)\end{matrix}$

For equation 2 above, T is the temperature of the gas in the storagevessel in Kelvin and P is the pressure of the gas in the vessels in MPA.For analysis purpose temperature can be assumed to be the ambienttemperature or worst case conditions for a pressure relief devicerelease (i.e., lowest operating temperature). The constants a_(i),B_(i), and C_(i) are defined in the (Lemmon, Huber, & Leachman, 2008)and are shown below in Table 3 below.

TABLE 3 i a_(i) b_(i) c_(i) 1 0.058 884 60 1.325 1.0 2 −0.061 361 11 1.87 1.0 3 −0.002 650 473 2.5 2.0 4  0.002 731 125 2.8 2.0 5  0.001 802374 2.938 2.42 6 −0.001 150 707 3.14 2.63 7 0.958 852 8 × 10⁻⁴ 3.37 3.08 −0.110 904 0 × 10⁻⁶  3.75 4.0 9 0.126 440 3 × 10⁻⁹ 4.0 5.0

Once the compressibility factor is determined, the density can becalculated using the ideal gas law, shown below as equation 3.

$\begin{matrix}{\rho_{mol} = \frac{P_{vessel}}{{ZRT}_{vessel}}} & (3)\end{matrix}$

For equation 3, P is the pressure of the gas inside the vessel in MPa, Zis the compressibility factor calculated from equation 2, R is theuniversal gas constant and T is the temperature of the gas inside thevessel in degrees Kelvin.

Equation 4 shown below can be used to convert the molar based densityfrom equation 3 to a mass based density (kilograms per cubic meter).

$\begin{matrix}{\rho_{{initial}\mspace{14mu} {Vessel}} = {\left( {\rho_{mol} \times {MW}_{hydrogen}} \right) \times \left( \frac{kg}{1000\mspace{14mu} {grams}} \right)}} & (4)\end{matrix}$

For equation 4, the molecular weight of hydrogen is equal to 2.01588grams per mole.

Initial Vessel Mass:

Equation 5 shown below can be used to determine initial mass. Equation 5provides the total mass of the hydrogen gas in the vessel at initialconditions in kilograms.

m _(Vessel initial)=ρ_(initial vessel) ×V _(vessel)  (5)

Orifice Area:

The orifice area can refer to the discharge area of the pressure reliefdevice when opened. Typically manufacturers provide the diameter of theorifice, but in the case that the diameter is not provided, one caninterpolate the orifice diameter based on flow curves or the device Cv.For the purposes of this example, it is assumed that the diameter of theorifice is given and equation 6 below can be used to calculate the area.In equation 6 below, A is the area of the orifice in meters squared, andR is the radius of the orifice diameter.

A _(ori) =πR _(orifice) ²  (6)

Critical Pressure:

Assuming steady state behavior, critical pressure is the state at whichthe upstream pressure is great enough to cause choked flow. Thisequation assumes that the downstream conditions of the orifice are closeto standard conditions. In equation 7 below, the inlet pressure is inkPa and k is the specific heat ration (Cp/Cv). Since hydrogen is anon-ideal gas, the specific heat ratio changes as a function of pressureand temperature. Look up tables can be used to determine the exact valuebased on the state of the system in respect to time.

$\begin{matrix}{P_{Crit} = {P_{i}\left( \frac{2}{k + 1} \right)}^{(\frac{k}{k - 1})}} & (7)\end{matrix}$

Orifice Equations:

The orifice equations can be used in the model to determine the real gasflow through the pressure relief device assuming the downstreamenvironment is at atmospheric conditions. Typically during a release,the flow through the pressure relief device will start in the chokedflow conditions, and then possibly transfer to the non-choked flowconditions depending on the orifice size. Generally for an ASME or DOTstamped PRD that is designed for fire or blocked flow, the PRD willreseat before the flow exits choked conditions.

Discharge Coefficient:

The discharge coefficient is a dimensionless number that is the relationof real flow vs. theoretical flow through an orifice. This number cantypically be determined imperially or by means of look-up tables. Manyforms of tables can be obtained from books, internet, etc. Typically thetables provide analysis as a function of the diameter ratio and theReynolds number. Table 4 shown below is an example of a typical table.For this analysis, the discharge coefficient is 0.60.

TABLE 4 Discharge Coefficient - c_(d) Diameter Ratio Reynolds Number -Re d = D₂/D₁ 1.00E+05 1.00E+06 1.00E+07 1.00E+08 0.2 0.6 0.595 0.5940.594 0.4 0.61 0.603 0.598 0.598 0.5 0.62 0.608 0.603 0.603 0.6 0.630.61 0.608 0.608 0.7 0.64 0.614 0.609 0.609 (Orifice, Nozzle and VenturiFlow Rate Meters)

Choked Flow Equation:

Equation 8 shown below can be used to determine the mass flow ratethrough an orifice in kilograms per second at choked flow conditions.For equation 8, inlet pressure is the pressure of the vessel, inletdensity is the density of the gas inside the vessel and k is the ratioof specific heats.

$\begin{matrix}{{\overset{.}{m}}_{c} = {{CA}_{ori}\sqrt{{kP}_{i}{\rho_{i}\left( \frac{2}{k + 1} \right)}^{(\frac{k + 1}{k - 1})}}}} & (8)\end{matrix}$

Non-Choked Flow Equation:

Equation 9 shown below can be used to determine the mass flow ratethrough an orifice in kilograms per second at non-choked conditions. Forequation 9, inlet pressure is the pressure of the vessel, inlet densityis the density of the gas inside the vessel, outlet pressure is thedownstream pressure of the orifice and k is the ratio of specific heats.

$\begin{matrix}{{\overset{.}{m}}_{nc} = {{CA}_{ori}\sqrt{2\; P_{i}{{\rho_{i}\left( \frac{k}{k - 1} \right)}\left\lbrack {\left( \frac{P_{o}}{P_{i}} \right)^{\frac{2}{k}} \times \left( \frac{P_{o}}{P_{i}} \right)^{\frac{k + 1}{k}}} \right\rbrack}}}} & (9)\end{matrix}$

Determining Choked Vs. Non Choked Flow Rates:

The actual flow rate through the orifice is dependent on if theconditions are choked or non-choked. This can be determined using thecritical pressure calculated by equation 7. Based on this, the following“if statement” can be used to determine the actual flow through theorifice.

IF P _(i) >P _(crit) Then {dot over (m)}={dot over (m)} _(c)

Else {dot over (m)}={hacek over (m)} _(nc)

Dynamic Analysis of a Pressure Relief Device Release

A PRD activation can result in mass being removed from the vessel as afunction of time. For this analysis, the basic equation for mass balancecan be used.

m _(vessel)(t)=∫_(t=o) ^(t=i) dm(t)−m _(vessel)  (10)

In respect to time=1 second, the equation can be represented by:

m _(vessel) _(Result) ={hacek over (m)}(t ₂ −t ₁)−m _(vessel)  (11)

For equation 10 and 11, dm is the mass removed by the orifice based onequations 8, 9, and 10 for the given time of i. Once the final vesselmass has been analyzed, a recalculation of vessel pressure based on theequations below can be done.

Determining Vessel Density at Time=i:

The volume of the vessel can be assumed static, therefore, the followingequation can be used to determine the resultant density after time i.

$\begin{matrix}{p_{i} = \frac{m_{vessel}\left( {t = i} \right)}{V_{vessel}}} & (12)\end{matrix}$

Determining Resultant Vessel Pressure After Time=i:

To determine the pressure at time=i, a formation of the NIST equation ofstate can be used, which is based on plot fit data from the NationalInstitute of Standards and Technology's Thermo-physical Web Book(http://webbook.nist.gov/chemistry/fluid/). For equation 13 below, T isthe temperature of the gas in Kelvin (assumed to be the same as theinitial condition at t=0), rho is the density of the gas at time=i, andP is the resultant pressure in kPa.

P=(T ^(cb) ×Bb×ρ)+((Ac+Bc×T+cc×T ² +Dc×T ³)ρ²)+((Ad+Bd×T+Cd×T ² +Dd×T³)ρ³)  (13)

The following constants shown in TABLE 5 below can be used in equation13.

TABLE 5 Constant Value Bb 4.7712 Cb 0.97583 Ac −13.129 Bc 0.10223 Cc−1.4128e−4 Dc  1.0819e−7 Ad 0.15844 Bd −1.9664e−4 Cd  7.9948e−7 Dd −6.2384e−10

Determining Orifice Conditions After Time=i:

Once pressure has been calculated as a result in the loss of mass fromthe storage vessel caused by the pressure relief device activation, onecan re-analyze the orifice equations 8, 9 and 10 to determine what theresultant mass flow rate can be.

Iterative Modeling:

Once the resultant orifice conditions are analyzed, time can be reset to0 and the next iteration is performed using equations 11, 12, and 13.This set can be repeated until the vessel pressure equals that of thepressure relief device or the vessel is empty.

Determining PRD Release Set Point:

Once the analysis is performed, analysis of the change in vesselpressure as a function of time can be done. Based on the information,the dP/dt value for the specific pressure relief device can bedetermined and programmed into the control system.

$\begin{matrix}{P_{Release} = {\frac{P}{t}\mspace{14mu} {storage}\mspace{14mu} {system}}} & (14)\end{matrix}$

Modeling for Effect of Additional Hydrogen Consumers or supplies

If the system has additional consumers (e.g., dispenser, fuel cell, orcustomer) or supply sources (e.g., hydrogen generator), equation 10 willhave to be modified to account for these variations in resultant mass ofthe vessel after time=i.

m _(vessel)(t)=∫_(t=0) ^(t=i)(dm(t)_(supply)+(dm(t)_(PRV)+dm(t)_(Consumer)))−m _(vessel)  (15)

For this example, the combination of all consumer(s) should not exceedthe maximum mass flow rate of the orifice inside the pressure reliefdevice. Otherwise, the alarm may activate when the PRD release conditiondoes not exist.

FIG. 3 shows a flow diagram for part of the control logic of controller170, according to an exemplary embodiment. The control logic isconfigured to detect if PRD 150 has been activated and if so, provideproper notification. Initially, when controller 170 is active, a systemcheck can be run to confirm inputs (e.g., instrumentation, valves) areoperating properly. The system check can comprise of confirming thevoltage, current, handshake signal, or other signal from theinstrumentation is within the expected range. If the signal is notwithin the programmed range, the system can fault, as shown in FIG. 3

Once controller 170 completes the system check, the process ofmonitoring storage tank 110 pressure can commence. Controller 170 canrecord the pressure reading of storage tank 110 pressure from pressuretransducer 160 at control loop time 1 (301). Subsequently, controller170 can then record storage tank 110 pressure again at control loop time2 (302). These two recorded pressure values can then be subtracted toget a differential pressure dP value. As shown in FIG. 3, the controlloop can be repeated and controller 180 can average these dP values overtime x resulting in a moving average dP/dt reading. As described above,during normal operation (i.e., gas 111 flowing through regulating device140) the dP/dt reading should be less than the calculated release ratefor a PRD 150 release.

As shown in FIG. 3, the average dP/dt reading can be compared to thecalculated PRD 150 release rate. The comparison function can be executedcontinuously or intermittently. Subsequently, if the dP/dt readingbecomes greater than the calculated pressure relief device release rate,the system will fault or alarm. Upon system fault or alarm, an immediateshutdown can be initiated. In addition, during the shutdown, controller170 can be configured to alert personnel that the system has experiencedan issue, which can prompt a proper response.

In other embodiments, following a system fault or alarm, controller 170can be confirmed to open or close valves or control other systemcomponents in a sequence designed to safely relieve the pressure fromthe tank. In addition, the sequence can be configured to bleed thepressure to a dump chamber (not shown) designed to receive the gas 111during an emergency shutdown. The dump chamber can limit the amount ofgas 111 that can be exhausted to atmosphere.

In other embodiments, controller 170 can be configured to receive asignal from regulating valve 140 indicating the current percent openposition of the valve. Controller 170 can use the current percent openposition to calculate a present dP/dt for regulating valve 140.Subsequently, the present dP/dt for the regulating valve 140 can befactored into the pressure relief device release rate set point, whichtriggers the system fault/alarm.

For example, if regulating valve 140 is significantly throttled andminimal flow is exiting through outlet passage 130, then an activationof PRD 150 would result in a release rate, which is almost entirely aresult of flow out exhaust 180. As a result, the measured dP/dt versusthe calculated release rate could be almost equal. Moreover, the marginof error for the pressure transducer, calculation rounding, orinstrumentation scaling could cause the measured dP/dt to be below thecalculated release rate in the final control logic comparison function,preventing the triggering of the system fault/alarm despite the actualactivation of PRD 150 (i.e., false negative). Therefore, by using thepresent dP/dt for regulating valve 140, controller 170 can adjust therelease rate set point appropriately to prevent false negatives.

Activation of PRD 150 can be a result of a variety of circumstances. Forexample, a control element failure, fire, blocked flow, or excess inletflow can all cause situations of over-pressurization. Controller 170 canbe configured to detect and then signal or notify the appropriatepersons or system(s). For example, controller 170 can be configured in acommercial storage application to send a signal to a fire panel upondetection so the proper emergency personnel can respond. In yet anotherembodiment, controller 170 can be configured to trigger an emergencyshutdown and initiate a switch over to an auxiliary hydrogen storagesystem upon detection of pressure relief device activation within theprimary hydrogen storage system.

Storage tank system 100 and the control method as described above can beused in many applications. For example, the system and method can beused for bulk station storage of pressurized gases (e.g., hydrogen orcompressed natural gas). The bulk storage could be a refueling stationfor mobile vehicles. Yet another example of an application could be onboard hydrogen storage vessels used with hydrogen combustion engines orproton exchange membrane (PEM) fuel cells in vehicles.

In other embodiments, the system and method as described above can beused to detect line break between the storage tank and downstream andupstream components. In other embodiments, the system and method asdescribed above can be used with a system receiving, storing, anddischarging a fluid, wherein the fluid is a compressible fluid.

In other embodiments, rupture disks can be utilized in place of pressurerelief device 150 as described above. It is contemplated that otherforms of pressure relief devices may also be utilized in conjunctionwith the present disclosure.

In other embodiments, a storage tank system can include a plurality ofstorage tanks and a plurality of pressure relief devices andcorresponding pressure transmitters. The pressure relief devices andpressure transmitters can be position in locations susceptible toover-pressurization due to control element failure. In systems utilizinga plurality of pressure relief devices a single controller can be usedor a plurality of controllers can be used and the plurality ofcontrollers can be configured to communication.

EXAMPLE 2

The system and method as described above can be utilized in a stationarybulk storage tank system. The storage tank system of the current examplecan be configured to receive, store, and distribute hydrogen. Thestorage tank system can comprise three storage tanks, wherein the firststorage tank is a high pressure tank, the second storage tank is amedium pressure tank, and the third storage tank is low pressure tank.The storage tank system can further comprise a pressure transmitter(PT_(H), PT_(M), or PT_(L)) and a pressure relief device (PRD). Thestorage tank system can further comprise a controller configured tomonitor the pressure of each of the pressure transmitters. Thecontroller can be configured to sample the pressure reading from eachtransmitter at 200 ms intervals. In parallel with the sampling thecontroller can be configured to calculate a rolling average of thepressure change for PT_(H), PT_(M), or PT_(L) over a period of 10seconds. The calculation of the rolling average can be executedutilizing the equations shown below for each of the each pressuretransmitter. The numbers of values listed in the matrix is not acomplete representation of the total number of values that would berecorded and calculated during a 10 second period with a polling rate of200 ms. The total number has been reduced to a small representativenumber.

Calculation for High Pressure Tank—PT_(H)

$\begin{matrix}{\begin{bmatrix}{\left( {{P\; 2_{Bank}} - {P\; 1_{Bank}}} \right) \div \left( {{T\; 2} - {T\; 1}} \right)} \\{\left( {{P\; 3_{Bank}} - {P\; 2_{Bank}}} \right) \div \left( {{T\; 3} - {T\; 2}} \right)} \\{\left( {{P\; 4_{Bank}} - {P\; 3_{Bank}}} \right) \div \left( {{T\; 4} - {T\; 3}} \right)} \\{\left( {{P\; 5_{Bank}} - {P\; 4_{Bank}}} \right) \div \left( {{T\; 5} - {T\; 4}} \right)} \\{\left( {{P\; 6_{Bank}} - {P\; 5_{Bank}}} \right) \div \left( {{T\; 6} - {T\; 5}} \right)}\end{bmatrix} = \begin{bmatrix}{P\; 1_{Calc}} \\{P\; 2_{Calc}} \\{P\; 3_{Calc}} \\{P\; 4_{Calc}} \\{P\; 5_{Calc}}\end{bmatrix}} & (16)\end{matrix}$

The average rate of change (dP) can be calculated by the followingequation:

Avg dP=Average(P1_(calc) ,P2_(calc) ,P3_(calc) ,P4_(calc),P5_(calc))  (17)

The average rate of change over time dP/dt can be calculated using thefollowing equation:

dP/dt=Avg dP*60  (18)

The above calculation can be executed for each storage tank pressuretransmitters simultaneously as shown above for the high pressure tank.

The controller can then take the minimum of the three dP/dt values forthe three storage tank pressure transmitters and compare this to the PRDrelease rate, which is a programmable variable based on the model of thePRD. If the minimum dP/dt value is greater than the PRD release ratethan the controller can trigger the alarm indicating a PRD activation.The controller can continuously execute the calculations and comparisonregardless of the filling state of the different tanks.

In an alternate embodiment, the system can utilize a pressure reliefdevice for each storage tank (PRD_(H), PRD_(M), or PRD_(L)). Therefore,the calculations as described above in determining the dP/dt for eachstorage tank can execute. However, instead of taking the minimum valueof the three tanks each value can be compared the PRD release rate forthe corresponding storage tank. This can allow the use of different PRDshaving a different set point for each storage tank.

The method of calculating the dP/dt and comparing the value to the PRDrelease rate as described above for the example can be utilized with astorage tank system having just a single tank or a plurality of tanks.In addition, the same method can be utilized for a storage tank systemconfigured for a mobile application, for example, onboard hydrogenstorage with in an automobile.

According to various embodiments, the system and method as describedabove can be utilized in storage tank systems storing gas at highpressure. The pressure of the gas within the storage tank can becompressed to pressure of about 5,000 psi, about 6,500 psi, about 7,500psi, about 10,000 psi, about 12,500 psi, or greater than about 15,000psi.

Other embodiments of the present disclosure will be apparent to thoseskilled in the art from consideration of the specification and practiceof the present disclosure herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the present disclosure being indicated by the followingclaims.

What is claimed is:
 1. A method of detecting activation of a pressurerelief device connected to a storage tank containing a pressurized gascomprising: calculating a pressure relief device release rate based on aset of inputs, wherein the set of inputs includes at least one of astorage tank volume, a pressure relief set point, an orifice size of thepressure relief device, a gas density, and a reseat point for thepressure relief device; monitoring the pressure within the storage tankand calculating a differential pressure reading over time; comparing thedifferential pressure reading over time to the pressure relief devicerelease rate; and detecting a pressure relief device activation based onthe comparison result.
 2. The method of claim 1, wherein the calculationof the pressure relief device release rate is based on choked flowequations for the pressure relief device.
 3. The method of claim 1,wherein calculating the differential pressure reading over time isrepeated and the differential pressure values are averaged over time. 4.The method of claim 1, wherein the method further comprises activating asystem alarm following detection of the pressure relief deviceactivation.
 5. The method of claim 1, wherein detecting the pressurerelief device activation based on the comparison result comprisesidentifying when the differential pressure reading over time is greaterthan or equal to the pressure relief device release rate.
 6. The methodof claim 1, wherein calculating the pressure relief device release ratefurther comprises factoring in the position of a regulating deviceconfigured to discharge the pressurized gas during normal operation. 7.A controller for detecting activation of a pressure relief deviceconnected to a storage tank containing a pressurized gas comprising aprocessor configured to receive a plurality of inputs of the pressurereading over time from a pressure transducer, calculate a differentialpressure reading over time and compare the differential pressure readingover time to a pressure relief device release rate determined based on aset of inputs, and based on the comparison the controller detectswhether a pressure relief device activation has occurred; wherein thepressure transducer is located upstream of the pressure relief deviceand the pressure within the storage tank is about greater than about6,500 psi.
 8. The controller of claim 7, further comprising a graphicaluser interface, a memory device, and a power source.
 9. The controllerof claim 8, wherein the set of inputs includes at least one of a storagetank volume, a pressure relief set point, an orifice size of thepressure relief device, a gas density, and a reseat point for thepressure relief device.
 10. A system configured to detect pressurerelief, comprising: at least one storage tank configured to contain agas; a pressure transducer configured to read the pressure of the gas; apressure regulating device configured to control the discharge of thegas during normal operation; a pressure relief device configured toactivate and discharge the gas to prevent an over-pressurization of atleast the storage tank; and a controller configured to receive thepressure reading over time from the pressure transducer, calculate adifferential pressure reading over time and compare the differentialpressure reading over time to a pressure relief device release ratedetermined based on a set of inputs, and based on the comparison thecontroller detects whether the pressure relief device has beenactivated.
 11. The system of claim 10, wherein the controller furthercomprises a processor, a graphical user interface, a memory device, anda power source.
 12. The system of claim 10, wherein the set of inputsincludes the storage tank volume, a pressure relief set point, anorifice size of the pressure relief device, the gas density, and areseat point for the pressure relief device.
 13. The system of claim 10,wherein the calculation of the pressure relief device release rate isbased on choked flow equations for the pressure relief device.
 14. Thesystem of claim 10, wherein calculating the differential pressurereading over time is repeated and the results are averaged over time.15. The system of claim 10, wherein the controller is further configuredto activate a system alarm following detecting the pressure reliefdevice activation, and the system alarm is configured to providenotification to at least the proper personnel or system to respond. 16.The system of claim 10, wherein the pressure relief device orificecross-sectional area is greater than the cross-sectional area of thepressure regulating device.
 17. The system of claim 10, wherein thecontroller is configured to factor in the position of a regulatingdevice configured to discharge the pressurized gas during normaloperation when calculating the pressure relief device release rate.